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Mineralogy and Chemistry of Desert Roses, Ayn Dar Area, Abqaiq, Eastern Province, Saudi Arabia Ahmed A

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Mineralogy and Chemistry of Desert Roses, Ayn Dar Area, Abqaiq, Eastern Province, Saudi Arabia Ahmed A Qatar Univ. Sci. J. (2002), 22 : 191 - 204 Mineralogy and Chemistry of Desert Roses, Ayn Dar Area, Abqaiq, Eastern Province, Saudi Arabia Ahmed A. Almohandis Department of Geology, College of Science, King Saud University, Riyadh, Saudi Arabia. 'J.oJ u~ ~ '-i~.J ,;;.l~ ~JJ-4 ~t.u.i ~>l~l.l.JJ.J ~.J '.lJ_jll ~ JS:.,a. 41~ ~\:i u!JJ-4 ~ ~>l~l.l.JJ.J ~...I.A '-:'~'-:'~~A·~~.¥'" ,,i:4!1 '-:'_;.! 'JI.l ~ ~ ~ Yro•• 4JA '-:'~ A>L.....o ~ ;;~ ~>l~l.l.JJ.J ~.J3.J .~.l_,......ll ~~'lli.ol.4 ,¥~' ~4rt...lll ~ ~~.J ,;;~ ~.lJ.J ~ ~ 'i$.lJ.J JS:.,a. ~ J.oJ u~ ~I$~ ~~.J ~~ u!JJ-4 4JA ~>l~l.l.JJ.J ~_,s::;:i.J A1. Acrt .L...:a...ll"~- - • ~· ..>£-~ - ~.,?"""'~'' v-:-:-... ·-'1 ul'.)~ 4k...l~ u-1.A _J-J·-J 1:r' ul.u..-.·- • .S~~~ ~lS~I ~.J ,~.#I.JI"~\.S:llj ,~1 ~lSi~JI.l ~ 'i14hi•.' ~>l~l.l.JJ_jl A.;w.:1.;JiuU.JS.tl~! 'i"~I.J ,~I.J 'I"~I.J 'l"~'i.:'li.J 'i"~~I.J ,.J.:...u.JI4,)A ;;~ .J:.lUA ~! ~~~4/"J:!-'-"l:i~I.J I".J:p_,.-ll ~lSi ·J~I.J ,JS:.::..i.li.J ~ ~.J '"l~l.l.JJ_jl ~ljj.a:i ~i ~~~ ¥~~~ ;;~ ~ Jl.l ~ ~ ~ 4:.JWI ~~~I.J A:!-oL...l.l ~! ~! ,;;~ ;;~ ~.lJ.J ul~ JS:.a. ~ J.o.;JI J>b ujS_,J ~ ,~1 ~ '-:'_;.! ~~ J.o.;JI ~ "'~' .l.JJ.J u!JJ-4 .;i~LJL..o..&S ....\..A.A..iA.t:..~.ll..a.l'~l ~~~La - l •. tlu~ ui•AI.tl-t:..l ...Ctl---ll ..;--- - - - _j.) - cu- - 'J 'j •. ~.,., V"'"J"' .J ~ ~~ ~~ Key Words: Sabkha - sand roses - gypsum - quartz -fibrous - hypersaline-nucleation. ABSTRACT Desert roses are crystals which usually take the form of rose petal. They have definite crystal shapes, and enclose sand grains. The desert roses were scattered along an area of about 500m2 in the Ayn Dar area, near Abqaiq, about 80km southwest of Ad Dammam, Eastern Province, Saudi Arabia. They are made up of gypsum crystals included sand grains in the from of rosettes, with distinctive petal morphology. Most quartz grains are cemented by gypsum crystals which show a fibrous structure. , • The main components of desert roses of Ayn Dar area are Si02 CaO and S03 Other oxides include Na20, K20 and small amounts of Fe, AI, Ti, Mg, Mn, Cr, Ni and P. High porosity and permeability in the Ayn Dar area permitted rapid ionic migration to promote the growth of desert roses. Desert roses crystals were grown in loose sand near the surface of the sabkha, and finally concentrated inside the sand as scattered large rose crystals. The large size of most crystals and the poikilitic inclusion of sand suggest accretion under stable hydrochemical conditions. 191 Mineralogy and chemistry of desert roses, Ayn Dar area, Abqaiq, Eastern Province, Saudi Arabia Introduction Gypsum occurs sometimes in rose like crystal aggregates similar to barite. Gypsum rose with inclusions of sand particles have been found at some localities in the world. Attractive specimens have been collect­ ed from Oklahoma, California in U.S.A. and from Mexico, Canada, and Egypt [1]. Many sediments contain large crystals or clusters of gypsum crystals. Some of these grow in a friable sand matrix in a fashion similar to barite and calcite. These crystals may include a large volume of sand. A well-known examples are the large crystal aggregates from the Laguna Madre, Texas [2]. Desert roses are not flowers, but crystals which take the from of rose petals. They have definite crystal shapes, and enclose as much as 60 percent of sand. The studied desert roses were scattered along an area of about 500m2 in the new Ayn Dar village (Fig. 1). The area was a small sabkha located close to the super­ tidal zone of the Arabian Gulf, near Abqaiq about 80km southwest of Ad Dammam, Eastern Province, Saudi Arabia. Desert roses were taken from the surface. However, some of them have been observed at a depth of 30-50cm below the surface. The sandy gypsum crystals (desert roses) range considerably in size, from 2-20cm in diameter interbed­ ded with unconsolidated sand. Some larger ones are in excess of 30cm in diameter (Fig. 2). The climate in the area can control the evaporate mineral facies and appears to be a critical factor in their presence and distribution [3 ,4]. The climate of the study area is typically desert. Mean temperatures range from 16°C in February to 37°C in August, with recorded extremes in the study area of about-2°C and 48°C. Rainfall is restricted to 4 months (Oct.- Jan.). The rainfall intensity is not uniform, but is of stormy nature. Evaporation rates are high, averaging over (76cm/month) during the summer [5]. The aim of this work was to investigate the mineralogy and chemistry of desert roses, and to shed some light on their formation. Geoglogical Setting A deposit of Quaternary sabkhah-related gypsum in the form of desert roses (sand roses) occurs in a small area, about 2km2 of Ayn Dar (Lat. 26°, Long, 49° 23). The area is filled with eolian sand (Fig. 2). The figure shows a geologic map ofAyn Dar and surrounding areas in the Eastern province, Saudi Arabian [6]. The sabkhah deposit, mainly gypsum was almost eroded away in the Ayn Dar area. The figure shows also other formations; Hofuf, Dam and Hadruck Formations. Where marine rocks occur, the Miocene and Pliocene succession can be subdivided into three forma­ tions which from the bottom up are Hadrukh, Dam, and Hofuf. Hadrukh rocks are mainly green, grayish green, and gray, but red, brown, white, and pink colours are also common. Marly sandstone, sandy marl, sandy clay, and sandy limestone make up most of the unit; chert is common at some levels, and minor amounts of gypsum are also present. The Dam Formation is pink, white, and gray marl and red, green, and olive clay with minor interbeds of sandstone, chalky limestone, and coquina. 192 Ahmed A. Almohandis The largest tingle exposure of the Hofuf Formation is superimposed on Ghawar field and almost per­ fectly mirrors the outline of that grant oil-filled anticline. The northern part of the structure is reflected in Hofuf beds at the surface by a prominent, dissected plateau capped by a resistant limestone layer. Hofuf rocks are imposed of conglomerate at the top and bottom of the Formation. White, in part impure sandy limestone. Moreover, Hofuf rocks contain arkosic sand with abundant orthoclase, microcline and quartz with igneous, metamorphic and sedimentary rock fragments. The source of the Hofuf rocks is the Arabian shield which consists of igneous, metamorphic and other rocks [7]. Materials and Methods Thin sections of desert roses were made in the rock laboratory of the Geology Department, college of science, King Saud University. They were prepared in the absence of water to prevent excessive dissolu­ tion of water-soluble minerals. The thin sections were ground under acetone to prevent dehydration of gyp­ sum to hemihydrite (bassanite ). These thin sections were then studied under the polarizing microscope. X-ray diffraction of about 15 samples was performed on a Siemens, D5000 diffractometer in the cen­ tral laboratory of college of science, King Saud University. The CuKa radiation was operated at 40kV and 35mA, with a curved graphite crystal monochromator, automatic divergence slit and 0.015-counting for 2 seconds on each step. On-line peak search and match program are based on [8]. The identification of the mineral phases and the assignment of the Miller indices to the spacing were done by a computer programs. Chemical analyses of desert roses was carried out in the laboratory of mineral sciences department, Smithsonian Institution, Washington, D.C., U.S.A.. All analyses were carried out using x-ray fluorescence techniques, except loss on ignition and so3 which were determined in the chemical laboratory of college of science, King Saud University, Riyadh, Saudi Arabia. The analytical, data reduction, and correction procedures of the XRF analyses are similar to those given by [8]. The accuracy of the XRF method for major elements was estimated to be ± 2% and for trace elements to be 5-10%. The loss on ignition (H20+) for all samples was calculated from weight loss after heating 2 gms of sam­ ple in vitreosil crucibles of900-1000°C for about 2 hours. Sulphate determinations (S03) were undertaken using the gravimetric technique [9 and 10]. Petrography Sandy gypsum crystals are made up of numerous discoid sub individuals closely intergrown in sub par­ allel and radial arrangements. As accretion proceeds, the gypsum crystals grow and become more complex until they assume the characteristics of rosettes, with distinctive petal morphology (Fig. 4). The mutual relations of the gypsum crystal and the manner of growth are the basis for classifying these structures as desert roses (rosettes) rather than crystal aggregates [2 and 11]. 193 Mineralogy and chemistry of desert roses, Ayn Dar area, Abqaiq, Eastern Province, Saudi Arabia Most gypsum crystals are colourless and belong to the satinspar crystalline variety. They occur as anhedral to subhedral aggregates, which show a fibrous structure. Extinction angle is parallel to (0 10) cleavage. Relief is low, refractive indices measured arena= 1.520, nB = 1.522, ny= 1.525. The gypsum crystals cleave readily on (101). However, traces ofweak cleavages on (011) and (100) can be seen under the microscope. Thin sections of desert roses (Plates 1 and 2) show that gypsum forms between 45-50% of the sections. Quartz forms the remaining ofthe sections. Most quartz grains are float­ ing and a very few of them have points of contact with their neighbours.
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